'Magic angle' trilayer graphene found to act as rare "spin-triplet" superconductor

Researchers at MIT and Harvard University have previously found that graphene can have exotic properties when situated at a 'magic angle'. Now, a new study by some of the members of the same team shows that this material could also be a "spin-triplet" superconductor one that isn't affected by high magnetic fields which potentially makes it even more useful.

"The value of this experiment is what it teaches us about fundamental superconductivity, about how materials can behave, so that with those lessons learned, we can try to design principles for other materials which would be easier to manufacture, that could perhaps give you better superconductivity," says physicist Pablo Jarillo-Herrero, from the Massachusetts Institute of Technology (MIT).

Superconducting materials are extremely sought-after: They can conduct electricity without losing heat, which could revolutionize our power grids, our portable gadgets, and much more if we can ever get them working at normal temperatures.

Usually, electrons in superconductors couple up in what are called Cooper pairs each with opposite spins (one up and one down), traveling through the material like linked passengers in an express train.

Rare types of superconductor are spin-triplet, however, which means the electrons have the same spin. Crucially, this means that a high magnetic field doesn't derail this imaginary express train, because the energy of both electrons shifts in the same direction.

Through a series of experiments, the team was able to show that magic-angle twisted trilayer graphene continued to behave like a superconductor at magnetic fields in excess of 10 Tesla three times higher than would be expected from a spin-singlet material.

What's more, the superconductivity disappeared and then came back as the strength of the magnetic field was increased.

"In spin-singlet superconductors, if you kill superconductivity, it never comes back it's gone for good," says physicist Yuan Cao from MIT. "Here, it reappeared again. So this definitely says this material is not spin-singlet."

More research is required to check the spin states of electrons in this special type of graphene. Early results seem to be very promising.

One area where spin-triplet superconductors could be helpful is in MRI scans: If these machines could operate under higher magnetic fields, they could produce much more detailed pictures. For now though, ultra-low temperatures in the lab are still required for the material to act as a superconductor.

The material and its rare properties also show promise for future research into quantum computing. A key issue for realizing the promise of practical, accessible quantum computers is improving their stability something that spin-triplet superconductors of a certain type could help with.

"We have no idea if our type is of that type," says Jarillo-Herrero. "But even if it's not, this could make it easier to put trilayer graphene with other materials to engineer that kind of superconductivity. That could be a major breakthrough. But it's still super early."

Posted: Jul 22,2021 by Roni Peleg